The present invention relates to a load sensing device for sensing a heavy load, for example, a load loaded on a big truck, or steel materials hanging from an overhead crane, or iron ore or steel materials or coil-shaped steel sheets transported in an ironworks and other heavy equipment employing factories. The present invention particularly relates to a load sensing device having a composition in which a smaller load can also be sensed. Such a load sensing device has a strain gauge sensing a strain occurring in a body of this device as a result of loaded material so that a load can be sensed.
A construction of one example of a load sensing device 1 in the prior art is described below with reference to FIG. 1. In FIG. 1, an outer part of the sensing device 1 is cut and removed for the sake of showing an internal construction thereof.
A manufacturing method of the load sensing device 1 is described below. A cylinder of height h is formed as a result of cutting an aluminum bar. Then a top of the cylinder is machined so that a top of a load bearing portion 2, to which a load is applied, protrudes. A plurality of vertical holes 4 are bored vertically between the load bearing portion 2 and a ring-shaped outer edge portion 3, in positions separated from each other by constant intervals. Thus, a plurality of strain portions 5 extending horizontally and radially from the load bearing portion are formed in the above mentioned intervals. The strain portions are formed so as to connect between the load bearing portion 2 and the outer edge portion 3 which is static when the load bearing portion is displaced due to application of a load. Strain occurs in the strain portions 5 when a load is applied to the load bearing portion from upward, vertically.
Strain gauges 6 are adhered on side walls of the strain portion. Strains in the strain portions, caused by a load applied to the load bearing portion, are sensed by the strain gauges 6.
The total number of strain gauges 6 is 8 because two gauges are provided on respective front and rear side walls of each of four strain portions 5. The strain gauges 6 are wired so as to compose a bridge circuit for eliminating error due to an expansion caused by heat. A connector 7 is connected to an output terminal of each strain gauge 6. A ring shaped covering sheet 8 covers each vertical hole 4.
A minimum sensitive load limit that can be sensed by the load sensing device 1 having the above mentioned construction is predetermined. This predetermined minimum sensitive load limit is, for example, 500 kilograms (kg). Minimizing of the minimum sensitive load limit can be achieved by an improvement of a sensitivity of the load sensing device 1 when a lighter load is loaded.
The sensitivity of the device 1 depends on the amount of strain occurring in the strain portions 5 when a load is applied to the load bearing portion 2. Thus a larger amount of strains caused by a certain applied load results in a higher sensitivity of the device 1 for the certain load.
The following two methods are apparent for this larger amount of strain: (I) to shorten the height h of the device 1 itself so as to reduce vertical thicknesses of the strain portion; (II) to enlarge the vertical holes 4 so as to reduce the intervals between the vertical holes 4, that is, to reduce the horizontal thicknesses of the strain portions 5.
However, the following problems occur if the method (I), that is thinning of the device 1, is applied. Strains occur with respect to a horizontal component of a load applied if the load includes a horizontal component, that is, if a direction of the applied load slants from the vertical direction. A first problem is that such strains as resulting from a component of an applied load other than a vertical component of the applied load may cause error of the load sensing.
A second problem is that, excessive shortening of the height h of the device 1 can cause a difficulty in attaching the strain gauges 6 on the side walls of the strain portions 5. Such problems define a limit of thinning of the device 1. Thus, it is difficult to improve the sensitivity of the device 1 by thinning of the device 1 itself.
On the other hand, the following problem in the manufacturing stage occurs if the method (II), that is, reducing the horizontal thicknesses of the strain portions 5, is applied. The vertical holes 4 are bored successively in the cylinder having the height h. Then the material of the strain portion 5 escapes toward an already bored vertical hole 4 as a result of the pushing force of a drill bit because the horizontal thickness of the strain portion 5 is too thin. Such pushing force of the drill bit occurs when a neighboring vertical hole 4 is being bored near an adjacent, already bored, vertical hole 4. This escaping of the strain portion 5 degrades boring accuracy.
As per the above mentioned reason, it is difficult to improve the sensitivity of the device 1, and thus the minimum sensitive load limit could not have been minimized in the device 1 of the prior art.